Best bivy bag(s) for cold wet weather

Is this with a overhead shelter or without? You can only go so cold before it isn't wet anymore.

I've been really digging my homemade Momentum 90 top & silnylon bottom bivy. It breathes well compared to other bivys I've tried.

That said, I could forgo some of the water resistance of the top in exchange for more breathability if needed. So far it hasn't been an issue.

I think total cost for the project was around $40 and that includes a 1/2 zip, draw string hood and such.

I've had success with an event bivvy (unishelter). it handles most conditions very well. while it could be used in prolonged wet weather I intend to put storm flaps on as the zippers don't inspire confidence. a friend had a non event WP/B version and it required the drying out of kit after only one night in the conditions you speak of. external fabric saturation I think would also likely be an issue with prolonged wet weather use with no chance to dry out.
I've used it year round though not for more than a couple of nights at a time. I would like to think it is possible to use it on an extended trip with down but I'm considering making a synth. quilt for those conditions/trips. I've had no condensation issues while using it in high humidity but I've not weighted my bag to discern if there has been moisture gain. used it successfully with VB liner in sub zero.
I'm thinking of a cape wrath trail trip this year. Scotland mostly low level with higher side trips. It is very wet in Scotland as most know. high level bivvying I'm confident of in with this bivvy in bad (wet/windy)conditions, though not for an extended period of time. lower level/less exposure complimented with tarp is my plan to increase comfort/versatility. if things get really crazy i'm confident that retreating to bivvy alone can be comfortable.
Apologies for digressing a bit here but for those interested I've dropped the weight (4oz) of the shelters pole. trekking pole with polystyrene covers over the tips can be used in an A frame inside the bivvy and achieves the same goal of interior space.
use with a tarp would be ideal. plan on using it with a spintex MLD in the future.
I really think the event works well in bivvies.
While heavy the 3 layer event has durability that I value for my usage.
I do plan to make my own lighter bivvy using something like momentum for committed tarp only use.
hope this helps a little
baz

Tom – I do NOT believe that there is a consensus on which bivy bag is best for minimizing condensation in cold wet weather at or near the dew point. Only some general theories that are highly dependent upon conditions. Pertex Quantum and Momentum 90 are two fabrics that are often talked about specifically for breathability, but they are NOT waterproof, they are only water-resistant by way of a DWR coating and tight weave. A variety of manufacturers use these as top fabrics. You will need something like a tarp overhead to handle rain. Sleeping bag inner and outer shell fabrics need to breathe too for it to matter. Also, don't breathe inside your bivy, and don't wear wet clothes to bed unless you are intentionally trying to dry them out (in which case you should plan on condensation).

As mentioned above, a lot depends on wheather or not you're using some kind of shelter and what kind of shelter. A single wall tent will result in terrible condensation, and it will be like getting rained on and a WR/B bivy may not get you through the night; however, a WR/B bivy will work in a double wall tent. For tarps and single-wall tents I'd want to use a WP/B bivy. I have one of each: an Equinox WR/B and a no longer made REI Ogre WP/B. The latter I have used stand alone in the winter with no shelter and had a great time. For a detailed discussion go to Backinglight.com>gear guides>shelters>bivy sacks.

Even GTX bivy bags have condensation in winter.

I'd go for an eVent bivy bag for the greater breathability while maintaining waterproofness.

Eric